A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
WO 2010/032878 discloses a lithographic apparatus comprising a sensor head configured to determine the position of a substrate table of the lithographic apparatus. The sensor head is arranged on a sensor arm extending under the substrate table. The sensor arm is rigidly mounted on the metrology frame of the lithographic apparatus. The substrate table of the lithographic apparatus comprises a holding device to hold a substrate in a holding plane. The substrate table further comprises a grid plate extending in a measurement plane parallel to the holding plane. The grid plate is arranged below the holding device, such that the sensor head arranged on the sensor arm can cooperate with the grid plate to measure a position of the substrate table.
A drawback of the position measurement system of WO 2010/032878 is that the position measurement system may be sensitive for dynamic movements and thermal influences, which may result in inaccurate position measurement. Inaccurate position measurement may result in exposure errors such as focus and overlay errors, and is therefore undesirable.